Telecom customers have been hit with another broadband outage tonight, just 13 hours after the day's first massive outage was remedied.

About 9.45pm their network was again hit, leading Telecom to apologise to their customers on Twitter - which those affected would have been unable to check.

"Sorry team, haven't had any service alerts through for broadband, will investigate and report back ... Our techs are investigating the broadband issue now team - really sorry, will update when we know more," a spokesperson tweeted.

As a result, their call centre is also taking a hammering, they said: "Our call centres obviously taking a few calls now, hence the long wait times. Apologies."

Internet users from Auckland to Canterbury reported they could not log on.

Spokeswoman Jo Jalfon said today's faults appeared to be related. Customers were having difficulty reaching Telecom's call centres because of a high volume of calls.

The earlier outage began at about 1am today and could have affected up to two-thirds of Telecom's broadband customers, or about 300,000 households, Jalfon said this morning.

But she said it was unlikely all of them would have tried to use the internet before Telecom began restoring services at 8am.

The outage also affected some eftpos services. Jalfon said that would have depended on the technology retailers were using.

Telecom said the fault was caused by a systems upgrade, which Jalfon later described as systems maintenance.

It was about 3am before technicians knew anything was wrong, she said. Jalfon was unsure when Telecom first alerted customers to the problem.

"Telecom seem to be running a curfew situation, no broadband between 9pm and 8am," he said. "Watching streaming movies is heaps harder with no Internet."

About half of Telecom's broadband customers lost service for about two hours on November 30 because of another early-morning fault that the company said occurred during scheduled maintenance on a computer server.